drm/doc: ci: Require more context for flaky tests

Flaky tests can be very difficult to reproduce after the facts, which
will make it even harder to ever fix.

Let's document the metadata we agreed on to provide more context to
anyone trying to address these fixes.

Link: https://lore.kernel.org/dri-devel/CAPj87rPbJ1V1-R7WMTHkDat2A4nwSd61Df9mdGH2PR=ZzxaU=Q@mail.gmail.com/


Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231025142441.745947-1-mripard@kernel.org